In hydrocarbon process plants, the formation of saline deposits in apparatuses cause severe problems for the safety of the operations of the various process units.
Said ammonium salt deposits are formed particularly in atmospheric distillation (topping) plants and in fluid bed catalytic cracking (FCC) plants, where the head vapours of the process units are often loaded with ammonia and hydrogen chloride.
Other plants that can present problems linked with the formation of ammonium and amine chlorides are the following:                SWS (Sour Water Stripper)        Gasoline desulphuration (Unifining)        HDS—Hydrogen Desulphuration System        
The most commonly formed salt is ammonium chloride. Deposits of solid ammonium chloride may form and collect on metallic surfaces and hinder the flow of gas, liquids and heat. Moreover, said deposits are highly hazardous for the development of corrosion mechanisms, known in fact as “under deposit”.
Ammonium chloride deposits can occur on distillation plates, in transport lines, on the surfaces of heat exchangers, on filters.
The deposit precipitates when the product of the partial molar pressures of the ammonia and of the hydrogen chloride exceeds the stability constant (Kd) of the ammonium chloride at system temperature.
In theory, changing the operating parameters could enable to prevent the ammonium chloride from depositing, by moving them from slightly above the equilibrium curve of the dissociation—Temperature constant to slightly below the curve.
This could be obtained by lowering the aforementioned product of the partial pressures by reducing the concentration of one or the other or of both ammonia and hydrogen chloride. However, in reality changing the operating conditions is nearly always infeasible.
For the removal of ammonia salt deposits, methods have been proposed which make use of amines that are stronger bases than ammonia, such as methoxypropylamine and monoethanolamine which form liquid salts under normal operating conditions. However, said salts are usually highly viscous and it has not yet been definitively demonstrated that such amines can overcome the problem of the onset of the corrosion phenomena described above. In fact, in some cases the salts of some amines used instead of ammonia can themselves be a source of solid or semi-solid deposits, and hence they must be removed just like ammonium chloride.
U.S. Pat. No. 4,793,865 describes the use of amines containing oxygen which react with the ammonium salt forming ammonia and an amine salt with a low melting point and/or a high affinity for water. The amine salt that is insoluble in hydrocarbons exits the unit in the liquid state, together with process liquids. Examples of these amines are dimethylethanolamine, monomethylethanolamine, methoxypropylamine, monoethanolamine, monoethylethanolamine, diethylethanolamine, propanolamine, and the like.
U.S. Pat. No. 5,387,733 describes the use of a non filming polyamine for the prevention and removal of ammonium chloride deposits, indicating dimethylaminopropylamine, diethylentriamine, ethylendiamine and tris-(2-aminoethyl)amine as polyamines.
Dispersants can act in various ways, from neutralising charges on the surface of the solid ammonium chloride, to destroying the crystal matrix, to the chemical displacement of ammonia from the crystal matrix.
Prior art dispersants are efficient to prevent a saline deposit from forming. Therefore, they are efficient preventive agents.
However, they are not equally efficient to remove the saline deposit, if it has formed.